You have an almost poetic idea there, storing the heat generated around your body into something keeping a garden thriving. Either way, I checked a cremation website and they said that their chamber heats up to between 1100 and 1800 degrees Fahrenheit. Keeping with that idea, you're gonna have a tough time reaching the same range. According to
http://www.enviroalternatives.com/fireQ&A.html (a marketing site), Oak burns between 900 and 1200 F, so that pushes the low end. On the upside, you're going to push the upper end of that temperature in a RMS. From there, you may be able to augment your fuel with something (I don't know what) that burns hotter. You'll have to design the whole thing to withstand the higher temperatures, but at least it sounds doable. I'd imagine that the body would be located after the barrel, where the stove pipe in thermal medium would usually be located.
Hope this helps, obviously not something I have
experience with!